This invention relates to the structure of and a method of producing a disk-shaped optical memory medium or a so-called optical disk on which laser light can be used for recording, playback and erasing.
Recently, so-called optical disks on which light can be used for recording, playback and erasing are being developed rapidly. An optical disk typically comprises a disk-shaped substrate with grooves formed thereon somehow in the form of a spiral or concentric circles such that a finely focused light beam can follow such a groove to record signals therein. Such a grooved disk may be produced by injecting a polycarbonate resin or an acryl resin into a mold having desired indentations. On the other hand, a method of producing grooved glass substrates as shown in FIGS. 3A-3E has been disclosed in Japanese Patent Publications Tokkai 60-195751 and 60-197960 according to which a photomask 8 with a desired pattern (such as grooves and a pit pattern) formed thereon is attached to a glass disk 7 coated with photoresist 7' (FIG. 3A) and it is exposed to a beam of ultraviolet light from above (FIG. 3B). Thereafter, the resist is removed from the exposed areas by a development process (FIG. 3C) and the desired pattern is transferred onto the glass substrate by an etching process (FIG. 3D). A desired optical memory can be obtained by removing the resist therefrom (FIG. 3E) and a film with recording, playback and erasing functions (such as optomagnetic materials with perpendicular magnetization characteristics and phase-change materials with amorphous-crystalline phase change characteristics) is formed on the surface.
With reference to FIGS. 3A-3E, if contaminants such as dust particles are deposited between the mask 8 and glass substrate 7 when they are attached together, the product may become defective because if the contact between the mask 8 and the glass substrate 7 is imperfect, the desired grooves and pit patterns cannot be properly transferred. Among such contaminants that become deposited between the mask 8 and the glass substrate 7, there is frequently a large quantity of fine glass powder in addition to fibers and what may generally be referred to as dust. Such glass powder usually comes about because edges of the glass substrate 7 break off easily. In other words, glass powder cannot be eliminated no matter what care is taken to clean the environment and to reduce the amount of dust during the production process. In order to reduce the occurrence of defective products during the production process, therefore, it is essential to reduce the generation of glass powder.